The remarkable progress in the technology of electronics in recent years has made possible downsizing and weight reduction of many electronic apparatuses, and it has resulted in a rapid advancement of cordless and portable operation of the apparatuses. With the advancement of these apparatuses, there is an increased demand for small and lightweight batteries (i.e., storage batteries, or secondary batteries) of high energy density, useful as power supplies for the electronic apparatuses. These batteries have been used as power supplies of numerous products including power sources of electric tools, backup power supplies, and the like. Nickel cadmium battery is one kind of these batteries (i.e., secondary batteries) that have been hitherto used widely as the power supplies. However, because of the increase in demand of high capacity, and worldwide concern of environmental problems, nickel hydrogen batteries have been developed recently as new products to replace the nickel cadmium batteries, and they are now spreading widely into the market.
In a battery package comprising any of these secondary batteries, however, there was a tendency in the past that a battery voltage decreases due to self-discharge and the like of the battery if it is left unused for a long period of time, in addition to a problem that a capacity of the battery decreases because the battery becomes inactive during repeated recharges and discharges, which eventually makes the battery unable to maintain a sufficient capacity. A cause of the inactivity of the battery is that the repeated recharges and discharges cause reduction of oxide in a positive electrode, and oxidization of hydroxide in a negative electrode. To clear the inactivity resulted from the above causes, the battery needs refreshing charge and discharge.
A number of methods were contrived to carry out the refreshing charge and discharge. To cite a few examples, one method is to give a display, which indicates a need to carry out a refreshing discharge by counting a number of recharges or discharges, and another method proposed is to increase the number to be counted according to a temperature of the battery and decreases a number of recharges before initiating the refreshing process since the battery tends to enhance the memory effect when the battery temperature is high (refer to an Unexamined Japanese Patent Application, Publication No. 2001-126776).
FIG. 3 is a block diagram of a battery package, prepared for the purpose of describing an example of such conventional methods of displaying a capacity of secondary battery used for the refreshing charge and discharge. In FIG. 3, packaged battery 101 (or, a battery package) used for the method of displaying a capacity comprises: secondary battery 103 of a type a dischargeable capacity of which decreases due to the memory effect as recharge and discharge are repeated; remaining capacity display device 113 for displaying a remaining capacity of the battery package by lighting up a corresponding number of indicator lamps among a plurality of them, or by using a liquid crystal display and the like; current detection circuit 114 for detecting a charging current and a discharge current of the battery; temperature sensor 104 placed in contact with secondary battery 103 for detecting a temperature of secondary battery 103; switch 106 composed of a semiconductor switching element such as an FET or a transistor, and connected between secondary battery 103 and output terminal 105; control circuit 107 for computing the remaining capacity and a timing of refreshing, in addition to controlling the switch 106; refreshing circuit 108 for discharging secondary battery 103 nearly completely to cancel the memory effect of the secondary battery 103; and refreshing display device 109 for displaying the refreshing operation.
Temperature sensor 104 in packaged battery 101 detects a temperature of secondary battery 103, and inputs a temperature signal to control circuit 107. Switch 106 is controlled by control circuit 107 in a manner that it turns on when secondary battery 103 needs charging, and it turns off when secondary battery 103 is fully charged to prevent overcharging of the secondary battery 103. In addition, switch 106 is controlled also by control circuit 107 in a manner to turn on when secondary battery 103 is discharged, but it is turned off again when secondary battery 103 is completely discharged to prevent the secondary battery 103 from being over-discharged.
Control circuit 107 is provided with counter 110 for counting a number of recharges of secondary battery 103, as well as operational circuit 111. Operational circuit 111 computes a timing to initiate a refreshing operation by comparing a counted value of counter 110 with a preset value, controls switch 106 by detecting a charging condition and discharging condition of secondary battery 103, and computing a remaining capacity from a charging current and a discharging current. Counter 110 counts the number of recharges by adding one to the counted value whenever recharging is made, and resets the counted value to zero when the secondary battery 103 is refreshed. Counter 110 also checks the charging condition by detecting a change in voltage across output terminals 105, when a charger (not show in the figure) is connected for charging secondary battery 103. Current detection circuit 114 is connected between secondary battery 103 and one of output terminals 105, and a voltage produced at both ends of this current detection circuit 114 is used to detect a charging current as well as a discharging current. Operational circuit 111 compares the number of recharges of packaged battery 101, or the counted value of counter 110, with the preset value, and informs of a timing of the refreshing operation when the counted value becomes equal to or exceeds the preset value. However, operational circuit 111 is controlled according to a program so that it determines the timing of refreshing operation after correcting the counted value of counter 110 based on the battery temperature, instead of comparing the counted value directly with the preset value, since the secondary battery 103 generally has a tendency of enhancing the memory effect when its temperature rises, and losing the memory effect when the temperature falls.
During the refreshing discharge in which the secondary battery is discharged from the fully charged condition to a final discharge voltage, the conventional method of this kind for displaying a capacity of the secondary battery comprises the steps of computing a full-charge capacity according to an integrated value of discharge current, correcting a remaining capacity by using the computed full-charge capacity, and further correcting the remaining capacity thereafter by using the counted value. Additionally, this method comprises the steps of counting a number of recharges of the secondary battery, and displaying a need of refreshing operation when the counted number of recharges reaches a predetermined number. The method further comprises the steps of detecting a battery temperature, and reducing the number of recharges to be counted before initiating a refreshing operation of the secondary battery.
In this method of counting a number of recharges, however, there was a drawback in which a need for the refreshing charge and discharge is not displayed when the battery become inactive as a result of it having been left unused for a long period of time, since the refreshing charge and discharged is programmed to be carried out according to the number of recharges or discharges, irrespective of the actual charge and discharge condition of the battery. To put it more concretely, consider an example of charge and discharge system which is designed to carried out a refreshing charge and discharge every after 50 times of recharging and discharging. After the battery has undergone 25 times of recharging and discharging process, for instance, it is still due to normal recharge and discharge without being subjected to another refreshing charge and discharge even if it is left unused for a long time without being recharged or discharged and becomes inactive, because the number of recharge counts has not reached 50 times. This is a problem to be resolved, since a resulting increase in an internal resistance of the battery due to enhancement of the memory effect and inactive condition of the battery makes it not capable of discharging a sufficiently large current, and prevents it from being used effectively.
The present invention was contrived to solve the above problems in the conventional method of carrying out the refreshing charge and discharge of the secondary battery contained in the battery package. It is thus an object of this invention to provide a technique which makes refreshing charge and discharge possible even for the secondary battery that has been left unused for a long time and become inactive, and to make the secondary battery effectively useful.